1. Field of the Invention
This invention relates to a radiation image recording and read-out apparatus for exposing stimulable phosphor sheets to a radiation passing through an object to have a radiation image of the object stored therein, exposing the stimulable phosphor sheets to stimulating rays which cause them to emit light in proportion to the stored radiation energy, and detecting and converting the emitted light into electric signals. This invention particularly relates to a radiation image recording and read-out apparatus in which the stimulable phosphor sheets are circulated and reused for recording radiation images.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet comprising the stimulable phosphor is first exposed to a radiation passing through an object to have a radiation image stored therein, and is then scanned with stimulating rays which cause it to emit light in proportion to the radiation energy stored. The light emitted from the stimulable phosphor sheet when the sheet is exposed to the stimulating rays is photoelectrically detected and converted to an electric image signal, which is processed as desired to reproduce a visible image having an improved quality, particularly a high diagnostic efficiency and accuracy. The finally obtained visible image may be reproduced in the form of a hard copy or may be displayed on a cathode ray tube (CRT). In this radiation image recording and reproducing system, the stimulable phosphor sheet is used to temporarily store the radiation image in order to reproduce the final visible image therefrom in a final recording medium. For economical reasons, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
Further, in a mobile X-ray diagnostic station such as a traveling X-ray diagnostic station in the form of a vehicle like a bus which is provided with a radiation image recording and read-out apparatus for use in the aforesaid radiation image recording and reproducing system and moves from place to place to record radiation images for mass medical examinations, it is disadvantageous to load the mobile X-ray diagnostic station with a large number of stimulable phosphor sheets, and the number of the stimulable phosphor sheets can be loaded on the mobile X-ray diagnostic station is limited. Therefore, it is desired to load the mobile X-ray diagnostic station with stimulable phosphor sheets which can be used repeatedly, store the radiation images of the objects in the stimulable phosphor sheets, transfer the electric image signals read out from the stimulable phosphor sheets into a recording medium having a large storage capacity, such as a magnetic tape, and circulate and reuse the stimulable phosphor sheets for further image recording and read-out operations, thereby to obtain the radiation image signals of many objects. Further, when image recording is conducted continuously by circulating and reusing the stimulable phosphor sheets, it becomes possible to increase the image recording speed in mass medical examination. This is very advantageous in practical use.
In order to reuse stimulable phosphor sheets as described above, the radiation energy remaining in the stimulable phosphor sheet after it is scanned with stimulating rays to read out the radiation image stored therein should be erased by exposure to light or heat as described, for example, in Japanese Unexamined Patent Publication No. 56(1981)-12599 or U.S. Pat. No. 4,400,619. The stimulable phosphor sheet should then be used again for radiation image recording.
From the aforesaid viewpoint, the applicant proposed in Japanese Patent Application No. 58(1983)-66730 a built-in type radiation image recording and read-out apparatus comprising:
i) a circulating and conveying means for conveying at least one stimulable phosphor sheet for recording a radiation image thereon along a predetermined circulation path,
ii) an image recording section positioned on said circulation path for recording a radiation transmission image of an object on said stimulable phosphor sheet by exposing said stimulable phosphor sheet to a radiation passing through said object,
iii) an image read-out section positioned on said circulation path and provided with a stimulating ray source for emitting stimulating rays for scanning said stimulable phosphor sheet carrying said radiation image stored therein in said image recording section, and a photoelectric read-out means for detecting light emitted from said stimulable phosphor sheet scanned with said stimulating rays to obtain an electric image signal, and
iv) an erasing section for, prior to the next image recording on said stimulable phosphor sheet for which the image read-out has been conducted in said image read-out section, having said stimulable phosphor sheet release the radiation energy remaining in said stimulable phosphor sheet, whereby said stimulable phosphor sheet is circulated through said image recording section, said image read-out section and said erasing section and reused for radiation image recording.
In the aforesaid radiation image recording and read-out apparatus, recording and read-out of radiation images can be conducted continuously and efficiently.
At the erasing section of the aforesaid radiation image recording and read-out apparatus, the stimulable phosphor sheet is exposed to light or heat to release the residual radiation energy from the sheet. In order to make the stimulable phosphor sheet reusable for image recording, it is necessary to expose the sheet to more light or heat as the level of the residual radiation energy becomes higher. (Because the amount of the residual radiation energy released increases as the light or heat exposure amount is increased.) Therefore, when the light or heat exposure amount at the erasing section is maintained constant, the exposure amount is adjusted so that the normally assumed maximum level of the residual radiation energy in the stimulable phosphor sheet can be released almost completely, i.e. to such as extent that no problem is caused by the residual radiation energy in the next image recording. On the other hand, when the light or heat exposure amount at the erasing section is controlled in accordance with the level of the residual radiation energy in the stimulable phosphor sheet for the purpose of decreasing the energy and time required for image erasing, the maximum exposure amount is adjusted to such a value that the normally assumed maximum level of the residual radiation energy in the stimulable phosphor sheet can be released almost completely.
However, in the case of a special radiation image recording, the stimulable phosphor sheet is exposed to a radiation in amount very much larger than usual. In the stimulable phosphor sheet exposed to a large amount of the radiation, the level of the residual radiation energy after image read-out often exceeds the normally assumed maximum value. Therefore, when the stimulable phosphor sheet carrying the high level of residual radiation energy is sent to the erasing section, the sheet should desirably be exposed to light or heat for long time to make the sheet reusable for image recording.
However, if the stimulable phosphor sheet exposed to the large amount of the radiation as described above is subjected to prolonged image erasing at the erasing section, the erasing section becomes a bottleneck in the flow of the stimulable phosphor sheet, and the image recording and read-out processing must be interrupted.